Calcium in plaque fluid (PF) is an important determinant of enamel saturation and may also be an important aggregating factor in the development of dental plaque. In spite of its importance little is known about the state of calcium in dental plaque, primarily because of the difficulty in studing the concentration of "free" and "bound" calcium in the small volume of PF available. Extensive measurements of PF calcium have recently been completed as part of our program to develop a micro analysis system for inorganic ions and organic acids in human dental plaque fluid (PF) before and after a cariogenic challenge 1-7. An important feature of this system is the ability to analyze samples recovered from single tooth sites so that the site specificity of cariogenic attack may be examined. Unfortunately, a recent study 8 indicated that some of the calcium micro electrodes used in the studies noted above, which employed a charged exchange sensor similar to the sensor employed in Radiometer "macro" electrodes, yielded significantly lower salivary calcium activities (28%) when tested against two types of protein-resistant macro Ca electrodes. However, a new type of micro electrode based on a neutral carrier sensor, which has been shown to be more resistant to biological artifacts, 9 appears to be free from interferences in saliva. Furthermore the previous work with suspect electrodes did not examine the relationship between pH and Ca activity at low pH. This information is essential for future work with whole plaque under cariogenic (acidogenic) conditions since it represents the response of PF in the absence of release from the plaque biomass. The purpose of this propsal is to develop reliable micro calcium electrodes for PF and to use these electrodes to study Ca activity and Ca-P saturation in single site resting PF as a function of pH.